2.1. Steric Relationship and Drug Action
Many organic compounds exist in optically active forms, i.e., they have the ability to rotate the plane of plane-polarized light. In describing an optically active compound, the prefixes D and L or R and S are used to denote the absolute configuration of the molecule about its chiral center(s). The prefixes d and l or (+) and (-) are employed to designate the sign of rotation of plane-polarized light by the compound, with (-) or l meaning that the compound is levorotatory. A prefix of (+) or d indicates that the compound is dextrorotatory. For a given chemical structure, these compounds, called stereoisomers, are identical except that they are mirror images of one another. A specific stereoisomer may also be referred to as an enantiomer, and a mixture of such isomers is often called an enantiomeric or racemic mixture.
Stereochemical purity is of importance in the field of pharmaceuticals, where many of the most prescribed drugs exhibit chirality. A case in point is provided by the beta-adrenergic blocking agent, propranolol, where the S-enantiomer is known to be 100 times more potent than the R-enantiomer. However, potency is not the only concern in the field of pharmaceuticals.